This invention relates to communications systems and methods, and in particular to radiotelephone communications systems and methods.
Cellular radiotelephone systems are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
FIG. 1 is a block diagram of a conventional cellular radiotelephone communication system. As shown in FIG. 1, cellular radiotelephone communication system 100 includes a plurality of cells 170 each of which includes a base station 120a-120n. Although two cells 170 are shown in FIG. 1, it will be understood that a large number of cells and a large number of base stations 120a-120n are generally included in the cellular radiotelephone communication system. Cellular radiotelephones 130 in the cells 170 communicate with the base stations 120a-120n. 
Cellular radio exchange 110 is connected to each of the base stations 120a-120n via a respective uplink 140a-140n and a respective downlink 150a-150n. Cellular radio exchange 110 controls communication among the cellular radiotelephones via the base stations. Cellular radio exchange also controls communications between the cellular radiotelephones 130 and wire telephones via a connection 160 to the Public Switched Telephone Network (PSTN). The design of cellular radiotelephone communication system 100 is well known to those having skill in the art and need not be described further herein.
Cellular radiotelephone communication systems are often installed for communications over a large geographic area using large cells, as evidenced by the cellular radiotelephone base stations that have now become a commonplace sight along highways. However, in densely populated cities, the cells may be small, covering several city blocks or less. Recently, it has also been proposed to provide cellular radio communication systems within a building in a DAMPS Wireless Office System (DWOS). In such a wireless office system, base stations may be located in spaced apart locations on one or more floors of an office building, to thereby provide wireless communications within the office building using conventional cellular radiotelephones.
In cellular systems, there may be conflicting requirements that may affect the architecture of the cellular radio exchange and the base stations. For example, it is generally desirable to produce low cost base stations and cellular radio exchanges. On the other hand, high quality voice communications should be provided by the cellular radiotelephone system.
More specifically, in a wireless office system, it is generally desired to maintain low cost of the base stations because many base stations are used in the system compared to the cellular radio exchange. In order to support high quality voice communication, it is also generally desired to use macro diversity, wherein radio frequency communications from a cellular radiotelephone are received by more than one receiver at a base station, to thereby allow diversity reception and improve radio link quality.
FIG. 2 illustrates a conventional cellular radiotelephone system with wideband transmissions between the base stations and cellular radio exchange. More specifically, in FIG. 2, cellular radiotelephone system 200 includes a base station 120xe2x80x2 and a cellular radio exchange 110xe2x80x2, wherein wideband signals are uplinked from the base station 120xe2x80x2 to the cellular radio exchange 110xe2x80x2 over uplink 140xe2x80x2, and wideband signals are downlinked from cellular radio exchange 110xe2x80x2 to base station 120xe2x80x2 over downlink 150xe2x80x2. It will be understood that although FIG. 2 illustrates only one base station 120xe2x80x2, many base stations conventionally are used. Moreover, more than one cellular radio exchange 110xe2x80x2 may also be used. The uplink 140xe2x80x2 and downlink 150xe2x80x2 may be provided by a radio frequency uplink and downlink, or a wired uplink and downlink, as is well known to those having skill in the art.
More specifically, the wideband architecture of FIG. 2 uses wideband digital distribution of digital Intermediate Frequency (Digital IF) signals, for example at a rate of 10-1000 megabits per second (Mbps). In the wideband architecture, the entire frequency band of received cellular radiotelephone communications is sampled and these samples are uplinked to the cellular radio exchange 110xe2x80x2 for processing. Thus, the base stations need only act as relays, so that the base stations can be simple and low cost. Signal processing complexity is placed in the cellular radio exchange 110xe2x80x2, where the central processing power can be shared and used efficiently to further decrease cost.
More specifically, referring to FIG. 2, base station 120xe2x80x2 includes a digital-to-analog converter (DAC) 202 that receives wideband digital distribution signals, for example at rates between 10-1000 Mbps, from downlink 150xe2x80x2. A first wideband channel filter 204 filters the analog signal. The filtered analog signal is provided to a radio frequency transmitter/receiver (transceiver) 230 that includes a first modulator 206, a transmit local oscillator (TXLO) 208, a power amplifier 210 and an isolator 212. The amplified radio frequency signal from power amplifier 210 is transmitted via antenna 224, to provide radiotelephone communications with a radiotelephone 130.
Radiotelephone communications that are received from a radiotelephone 130 at antenna 224 are routed to low noise amplifier 214 by isolator 212 and are then down-converted from radio frequency by a second modulator 216 using receive local oscillator (RXLO) 218. The output of second modulator 216 is provided to second wideband channel filter 220 and to an analog-to-digital converter (ADC) 222. The digitized output of analog-to-digital converter 222 is uplinked to cellular radio exchange 100 via wideband uplink 140xe2x80x2. Accordingly, similar to downlink 150xe2x80x2, uplink 140xe2x80x2 is a wideband digital uplink, for example at rates between 10-1000 Mbps.
At the cellular radio exchange 110xe2x80x2, the wideband uplink 140 is received by digital channelizer 252 and separated into two or more digital channels that are provided to a demodulator 250. Demodulator 250 includes at least two synchronizer/downsamplers 254a and 254b, that synchronize and downsample the digital channels created by the digital channelizer 252. The synchronized and downsampled signals are then provided to a diversity combiner/equalizer 256 that can perform macro-diversity combining. The output of the diversity combiner/equalizer 256 is provided to a channel decoder 258, the output of which is provided to speech decoder 260. The decoded speech is then provided to a processor 270, for example at 64 kilobits per second (Kbps) per call (i.e. per radiotelephone communication), which can perform control functions, such as routing signals to PSTN 160 and other base band and signal processing functions such as echo canceling.
When a signal is received by processor 270 that is to be transmitted to a radiotelephone 130, the signal is provided to speech coder 272 (for example at 8 Kbps/call) and channel decoder 274. Channel coder 274 may provide I/Q symbols at 48.6 Kbps per call. The speech coded signal is then provided to modulator 276 to produce the wideband signal that is downlinked to base station 120xe2x80x2 over downlink 150xe2x80x2. The overall design of base station 120xe2x80x2 and cellular radio exchange 110xe2x80x2, as well as the design of the individual components thereof, as shown in FIG. 2 are well known to those having skill in the art and need not be described in detail herein. An example of wideband architectures is described in U.S. application Ser. No. 08/540,326, filed Oct. 6, 1995 al., entitled Distributed Indoor Digital Multiple-Access Cellular Telephone System now U.S. Pat. No. 5,903,834, issued May 11, 1999, the disclosure of which is hereby incorporated herein by reference.
Accordingly, in a wideband system as described above, a low-cost architecture can be provided because the base stations 120xe2x80x2 are relatively simple, while the primary processing functions are performed in the central cellular radio exchange 110xe2x80x2. Moreover, voice quality can be high because diversity demodulation may take place in the cellular radio exchange 110xe2x80x2. Unfortunately, a wideband architecture of FIG. 2 may require the use of high speed transport technology, for example in the range of 10-1000 Mbps, over moderately long distances up to 100 meters or more. It may be difficult and costly to transport samples from the base station to the cellular radio exchange using conventional technology such as fiber optics, high speed copper and xDSL. In many applications, the added cost from the sample transport system can make the wideband architecture unattractive.
FIG. 3 illustrates a narrowband architecture for a cellular radiotelephone communication system. As shown in FIG. 3, narrowband cellular radiotelephone communication system 300 includes a narrowband downlink 150xe2x80x3 that downlinks narrowband digital information, for example at 16.1 Kbps per call and a narrowband uplink 140xe2x80x3 that uplinks narrowband digital signals at 16.1 Kbps/call from the base station 120xe2x80x2 to the cellular radio exchange 110. Compared to FIG. 3, modulator 276 is moved from the cellular radio exchange 110xe2x80x3 to the base station 120xe2x80x3 and filters 204xe2x80x2 and 220xe2x80x2 are narrowband filters. Moreover, demodulator 250xe2x80x2 is moved from the central cellular radio exchange 110xe2x80x2 to the base station 120xe2x80x3. As also shown in FIG. 3, demodulator 250xe2x80x2 includes a single synchronizer/downsampler 245xe2x80x2 and an equalizer 256xe2x80x2. Since separate diversity channels have not been channelized, macro diversity combining is generally not performed in base station 120xe2x80x2. Macro diversity can be performed in the cellular radio exchange by adding radio quality information to the narrowband signal. Unfortunately, this type of macro diversity may have the performance of a switched diversity receiver, which may be less than the performance of in-phase addition diversity in a wideband signal. The overall design of base station 120xe2x80x3 and cellular radio exchange 110xe2x80x3, as well as the design of the individual component thereof, as shown in FIG. 3 are well known to those having skill in the art and need not be described in detail herein.
The narrowband architecture as described in FIG. 3 is widely used in many cellular radiotelephone architectures including DECT, GSM, PDC and AMPS/DAMPS. The narrowband architecture allows standard ISDN, T1 or E1 narrowband uplinks 140xe2x80x3 and downlinks 150xe2x80x3 to be used between the base stations 120xe2x80x3 and the cellular radio exchange 110xe2x80x2. Accordingly, samples may be transported at low cost. The complexity of the signal processing in the cellular radio exchange 110xe2x80x3 may also be reduced compared to cellular radio exchange 110xe2x80x2 of FIG. 2, so that the cost thereof may be reduced.
Unfortunately, the narrowband architecture shifts processing from the central radio exchange 110xe2x80x3 to the base stations 120xe2x80x2, which may make the base stations more expensive. Since there are generally many more base stations 120xe2x80x3 than cellular radio exchanges 110xe2x80x3, the overall cost of the narrowband cellular radiotelephone communication system 300 may increase dramatically.
Moreover, the narrowband architecture generally does not use macro diversity. The lack of macro diversity can further increase costs due to the increase in the number of base stations that may be needed to improve coverage. The lack of macro diversity can also decrease voice quality due to poor radiotelephone communication transmissions between the radiotelephones and the base stations.
Accordingly, notwithstanding the use of wideband and narrowband architectures between the cellular radio exchange and the base stations of a cellular radiotelephone communication system, there continues to be a need for cellular radiotelephone system architectures that can provide reduced cost, high quality and diversity processing.
It is therefore an object of the present invention to provide cellular radiotelephone communication systems and methods that can include low cost base stations.
It is another object of the present invention to provide cellular radiotelephone communication systems and methods that can include low cost base stations without the need for expensive uplinks and downlinks between the base stations and the cellular radio exchange.
It is still another object of the present invention to provide cellular radiotelephone communication systems and methods that can include low cost base stations without the need for expensive uplinks and downlinks between the base stations and the cellular radio exchange, and which also can perform diversity processing, to thereby improve voice quality of cellular radiotelephone communications.
These and other objects are provided according to the present invention, by cellular radiotelephone communication systems and methods that downlink digital coded speech from the cellular radio exchange to the cellular radiotelephone base station and that uplink downsampled (i.e. undersampled relative to the Nyquist rate of two samples per symbol) radiotelephone signals from the cellular radiotelephone base station to the cellular radio exchange. The downsampled radiotelephone signals are demodulated at the cellular radio exchange and the digital coded speech is modulated at the cellular radiotelephone base station. Diversity demodulation may be used at the cellular radio exchange, to diversity demodulate the downsampled radiotelephone signals at the cellular radio exchange.
By downlinking digital coded speech from the cellular radio exchange to the cellular radiotelephone base station, and uplinking downsampled radiotelephone signals from the cellular radiotelephone base station to the cellular radio exchange, narrowband uplinks and downlinks may be provided between the base stations and the cellular radio exchange, to thereby reduce cost. Moreover, since demodulation takes place at the cellular radio exchange rather than at the base stations, low cost base stations may be provided. However, since downsampled radiotelephone signals are transmitted from the base stations to the cellular radio exchange, diversity demodulation may be performed at the cellular radio exchange, to thereby improve the voice quality of the cellular radiotelephone communications. Thus, for example, downlinking and uplinking may be performed at rates that are an order of magnitude of Kbps per radiotelephone communication. Accordingly, low-cost, high-quality cellular radiotelephone communication systems and methods may be provided.
In preferred embodiments of the present invention, uplinking downsampled radiotelephone signals from the cellular radiotelephone base station to the cellular radio exchange is obtained by downsampling received radiotelephone signals at the cellular radiotelephone base station. Downsampling is performed in a narrowband receiver at the cellular radiotelephone base station. Downsampling is preferably performed by first oversampling (relative to the Nyquist rate) the received radiotelephone signals at the cellular radiotelephone base station and using these samples to perform synchronization. Thereafter, the synchronized received radiotelephone signals are undersampled (relative to the Nyquist rate) and the undersampled received radiotelephone signals are uplinked to the cellular radio exchange.
Cellular radiotelephone communication systems according to the present invention include a plurality of cellular radiotelephone base stations that communicate with a plurality of cellular radiotelephones and that uplink downsampled radiotelephone signals. A cellular radio exchange receives the uplinked downsampled radiotelephone signals and downlinks digital coded speech to the plurality of cellular radiotelephone base stations. The cellular radio exchange also includes a demodulator that demodulates the downsampled radiotelephone signals and the cellular radiotelephone base stations include a modulator that modulates the digital coded speech. The cellular radio exchange also includes a decoder that is responsive to the demodulator, to decode speech from the demodulated downsampled radiotelephone signals. The cellular radiotelephone base stations include a radio frequency transmitter/receiver (transceiver) that is responsive to the modulator, to transmit the modulated digital coded speech as a radio frequency signal that is received by the cellular radiotelephones.
Preferably, the demodulator in the cellular radio exchange is a narrowband diversity demodulator. The cellular radiotelephone base station also preferably includes a narrowband downsampler that downsamples received radiotelephone signals. The downsampler preferably comprises an oversampler that oversamples the received radiotelephone signals to establish synchronization and an undersampler that is responsive to the oversampler to undersample the synchronized received radiotelephone signals. Accordingly, the uplink from the base stations to the cellular radio exchange is relatively narrowband, to thereby allow low cost uplink. However, the uplink is preferably undersampled synchronized radiotelephone signals, so that diversity demodulation may be performed in the cellular radio exchange.
Accordingly, a low cost uplink may be provided, while still retaining much of the processing power in the cellular radio exchange. Diversity demodulation may be used for improved voice quality. It will be understood that the present invention may be provided as communications methods between a cellular radiotelephone base station and a cellular radio exchange, communications methods by a cellular radiotelephone base station, communication methods by a cellular radio exchange and may also be provided as cellular radiotelephone communication systems, cellular radiotelephone base stations and cellular radio exchanges.